The ultimate objective of this research is to determine the mechanism by which virulent Agrobacterium tumefaciens transforms plant cells into autonomously growing tumerous-tissue. Axenic tumor cells grown in vitro produce novel amino acids (octopine or nopaline, depending upon the inciting bacterial strain). In addition, such tumor cells proliferate without exogenously supplied growth hormones (auxin and cytokinin) which normal plant tissue requires for growth in vitro. In general, tumor lines continue to exhibit these two characteristics despite repeated subculturing over many decades. This seemingly heritable alteration, produced by the bacteria during a brief contact with the plant tissue, has been ascribed by many investigators to gene transfer from bacterium to plant. A new slant on this DNA-transfer model was provided when Schell's group discovered that virulent Agrobacterium strains all contained large covalently closed circular plasmid DNA while many avirulent strains did not. They suggested, from this correlation, that virulence is conferred by large plasmids in A. tumefaciens. This hypothesis, which has been proven true for a large number of strains, is now accepted by most investigators. Proof for the hypothesis came from experiments in this laboratory (see Watson et al. 1975) and also in Schell's laboratory. We found that virulent A. tumefaciens strain C-58, which upon growth at elevated temperature was known to lose virulence, concomitantly lost a large plasmid. The "cured" plasmidless strain has subsequently been used in conjugational crosses both in planta (see Watson et al., 1975) by a published procedure and in vitro (Levin et al., submitted; M.-D. Chilton, unpublished results). In every case where virulent exconjugants are isolated, they have been found to contain a large plasmid like that of the donor parent strain. BIBLIOGRAPHIC REFERENCES: Hanson, Richard S. and Chilton, M.-D. On the question of integration of Agrobacterium tumefaciens DNA by Tomato Plants, J. Bacteriol. 124, 1220 (1975). Kleinhofs, A., Eden, F.C., Chilton, M. -D., and Bendich, A.J., On the question of the integration of exogenous bacterial DNA into plant DNA. Proc. Nat. Acad. Sci. U.S. 72, 2748 (1975).